Ultrafast laser energy deposition in copper revealed by simulation and experimental determination of optical properties with pump-probe ellipsometry

Ultra-short pulsed lasers offer a great potential in precise and efficient material processing. Experimental and theoretical studies on efficiency of laser material processing from metals have demonstrated a high degree of dependency on the laser pulse duration. Within these studies, the investigation of the transient energy deposition in material takes a great significance for the thermal and mechanical material response after laser irradiation. The scope of this study was to investigate the ultra-fast energy deposition in a copper metal during the irradiation with a 680 fs ultra-short laser pulse at a 1056 nm and a 528 nm wavelength. For this purpose, a numerical analysis of the laser-matter interaction was performed by using the optical Drude critical point (DCP) model and thermal two temperature model fully coupled with thermoelasticity theory (2T-TE). The DCP model was incorporated into the 2T-TE to simulate the ultra-fast laser energy deposition and optical material response of copper. For comparison with experimental data a pump probe ellipsometry set-up was used. The pumpprobe ellipsometry set-up combines the high temporal resolution of pump-probe technique and ellipsometric measurements of optical indices. It was found numerically that a dynamic change on re ectivity and optical penetration depth at 1056 nm is mainly induced by a rapid temperature increase. In contrast at the irradiated wavelength of 528 nm the laser pulse absorption is mainly described by the interplay of the the interband excitation and intraband heating of conduction band electrons The time resolved simulation of optical indices (n, k) confirms the temporal experimental observation of refractive index and extinction coefficient within the first 10 ps from pump-probe ellipsometry set-up.